Advisory signs are widely used on highways to provide a variety of messages to motorists, such as traffic control information at an intersection, highway condition, a pedestrian cross-walk, the presence of a hazard such as an excavation or a road construction barricade and the like.
For night-time visibility, such advisory signs are typically arranged to reflect light from a highly reflective surface, for example, a reflector surface comprising an array of corner reflectors typically colored or utilized in conjunction with a color filter. The reflecting surface is typically arranged in the shape of symbols and/or words to provide a specific message or may be triangular or simply circular as in the case of reflectors commonly used to draw attention to the edges of a driveway or to the presence of a utility pole close to a roadway.
In order to draw the attention of a motorist more effectively, such signs are sometimes provided with flashing lamps or else flashing lamps are themselves arranged in the form of the message words or symbols. It is recognized that signs incorporating such lamps have substantially higher first cost and require a source of power for operation; furthermore, periodic maintenance is required for lamp replacement. It is also recognized that these requirements generally restrict the deployment of such illuminated signs.
Various arrangements are known for attempting to provide a flashing sign without the above-mentioned drawbacks. Thus, U.S. Pat. No. 4,893,903, "FLASHING ADVISORY SIGN", issued Jan. 16, 1990 (Thakar et al.), and herein incorporated by reference, discloses a flashing advisory sign having an electrically operable shutter positioned between a message sign and a viewing region. The optical shutter is made of a liquid crystal plastic material having a substantially opaque state, in which the message sign is not visible from a viewing region, and a transparent state in which the sign is visible. The state of the optical shutter is controlled by a flashing power supply circuit, the transparent state being achieved when power is applied to the optical shutter and the substantially opaque state being achieved when power is not applied. Another sign arrangement with a liquid crystal type shutter is described in British Patent Specification No. 1,457,531, "IMPROVEMENTS IN OR RELATING TO SIGNS", published Sep. 16, 1974 (Prior et al.)
While the power supply requirement of a liquid crystal shutter and the associated driving circuit arrangement are substantially less than is required for operating lamps, a source of a power supply is nevertheless still required, for example, even where the device is used as an embedded highway lane marker.
Optical shutters of this general type are known in the art, for example, as disclosed in U.S. Pat. No. 4,475,031, "SOLAR POWERED SUN SENSITIVE WINDOW", issued Oct. 2, 1984 (Mockovciak, Jr.), herein incorporated by reference, which describes a self-contained sun sensitive window made up of liquid nematic crystals sandwiched between two transparent conductors and powered directly by a solar cell. The solar cell is located so that sunlight incident upon it is representative of sunlight incident upon the window. An increase of light intensity upon the solar cell reduces the transparency of the window, resulting in a constant intensity behind the window. Mockovciak, Jr. describes two general types of liquid crystal film, one of which is normally opaque and becomes transparent upon the application of an electric field and the other of which types transmits light in the normal state and retards light transmission upon the application of an electric field.
Other applications of such shutters are described in the following, herein incorporated by reference: U.S. Pat. No. 4,791,418, "SIGNAL LIGHT", issued Dec. 13, 1988 (Kawahara et al.); U.S. Pat. No. 4,690,508 Sep. 1, 1987, "LIQUID CRYSTAL CLOSED LOOP CONTROLLED MIRROR SYSTEMS", issued Sep. 1, 1987 (Jacob); U.S. Pat. No. 4,071,912, "LIGHT FILTER FOR WELDER'S MASK", issued Feb. 7, 1978, (Budmiger); U.S. Pat. No. 3,614,210, "LIQUID CRYSTAL DAY/NIGHT MIRROR", issued Oct. 19, 1971 (Caplan). French patent application no. 74.15031, "VARIABLE TRANSPARENCY SCREEN ESPECIALLY FOR MOTOR VEHICLES", opened to public inspection Nov. 29, 1974 (FIAT), discloses a variable transparency screen for motor vehicles to shield against the glare of oncoming headlights. A number of related patents are referenced in the publication "LIQUID CRYSTAL DEVICES (STATE OF THE ART REVIEW, Vol. 7)", published by Optosonic Press, Box 883, Ansonia Post Office, New York, N.Y. 10023; 1973 (Library of Congress Catalog Card No. 73-78006).